There are occasions on which it is necessary to hold specimens in a gaseous environment during imaging in a scanning electron microscope. Examples include imaging of biological or non-conductive specimens. The gaseous environment inhibits the evaporation of moisture from the biological specimens and dissipates surface charges from the non-conductive specimens, which charges would otherwise accumulate to the detriment of image resolution.
Use of a gaseous environment to amplify a secondary electron signal obtained during imaging of a specimen is described in U.S. Pat. No. 4,785,182 (Mancuso et al.), European Patent EP 330 310 (Electroscan Corp.) and PCT Application No. PCT/GB97/03136, in which secondary electrons released by the specimen as a result of interaction with a scanning electron beam are accelerated through the gaseous environment, some of the secondary electrons colliding with the gas molecules making up the gaseous environment. The collisions ionise the gas molecules to release further electrons, some of which collide with other gas molecules to release yet further electrons. The avalanche of electrons released by the ionisation of the gas molecules as a result of such collisions in effect provides the amplification of the secondary electron signal.
PCT/GB97/03136 describes apparatus which detects the amplified secondary electron signal by using a photodetector and photomultiplier to detect photons emitted as a result of the collisions of the electrons with gas molecules.
However, as the gas pressure is increased, the strength of the secondary electron signal is reduced since the gas reduces the proportion of the primary beam which reaches the specimen. This correspondingly limits the maximum gas pressure at which the microscope can create an image.
In scanning electron microscopes in which the specimens are held in a substantially evacuated environment during imaging, it is well known to detect the secondary electrons using an Everhard-Thornley detector. In such a detector the secondary electrons are accelerated towards a scintillator to convert them into photons which are transmitted to a photomultiplier.
The Everhard-Thornley detector has been found to be relatively sensitive, but uses a high voltage to establish an electrostatic field in which the secondary electrons are accelerated towards the scintillator. In a gaseous environment, too high a voltage would be sufficient to cause a discharge through the gas.